Portable computers typically can be powered from the public power grid using an ac-to-dc converter. They can also be powered by rechargeable batteries, so that if a power grid outlet is not nearby or the adapter is unavailable, the computer can still be used.
The batteries can be recharged from the ac power grid either by removing the battery housing from the computer and placing it in a charge receptacle, or during use of the computer when the computer is plugged into the ac power grid. However, if the public power grid is not available and the battery discharges, the user must remove the battery and replace it with another, fresh battery if he desires to continue to use the computer. As recognize herein, swapping batteries in this way requires a complete shutdown of the computer and subsequent boot on the new battery, which is inconvenient.